Amplifiers that have a linear input-output transfer curve are desirable in low distortion applications. In real practice, however, amplifiers have a nonlinear transfer curve. A typical transfer curve is one in which the slope of the curve diminishes for large input amplitudes. This produces gain compression, which results in distortion at the output of the amplifier. FIG. 1 shows a typical input-output transfer curve 12 of a power amplifier 10 illustrating gain compression. When an amplifier 10 with the characteristic of FIG. 1 is used to amplify a signal Vi, the output Vo is distorted. For example, if the input Vi is just a single-tone sinusoidal signal, that is, a signal with only one frequency component, f1, the output Vo instead will be a signal that will contain harmonics at integer multiples of the original input frequency. In a second example, if the input Vi is a two-tone signal with just two close-by frequency components, f1 and f2. The output Vo will be a signal that will contain harmonics and also intermodulation components, some of them will be close to the original frequencies f1 and f2. The distortion is determined by the height of the harmonic and intermodulation components. The distortion can be reduced by linearizing the amplifier 10.
Future power amplifier systems will require high bandwidth, high efficiency, and low harmonic content. Unfortunately, these goals are normally mutually exclusive. The presence of harmonics in amplifiers in real systems leads system designers to utilize constant envelope modulation, narrow bandwidth, and filtering to address individual system functions. In a wideband, RF multi-beam, multi-function system, the presence of multiple heterogeneous signals makes such an approach impractical. Backing-off on power levels is a commonly used but extremely inefficient method for improving linearity. Multi-beam, multi-function beams inevitably lead to nodes and anti-nodes in the signal that need to be faithfully reproduced to eliminate intermodulation products.
Methods for improving linearity include feed forward, feedback correction, and predistortion. Simple analog methods are subject to component mismatch and thermal drift, and commonly only address lower order harmonics of narrow band signals. The nonlinearity in the transfer curve 12 can be corrected by adding a predistorter amplifier 20 at the input of the amplifier 10, as shown in FIG. 2.
FIG. 2 shows the principle of predistortion. The predistorter amplifier 20 modifies the input signal Vpd to cancel the nonlinearity of the output signal Vo of the main amplifier 10. For example, if the main amplifier 10 exhibits gain compression for large amplitudes the predistorter 20 should exhibit gain expansion for larger input amplitudes. The predistorter 20 has a gain expansion as illustrated in transfer curve 25. Its output Vpd is connected to the input of the power amplifier 10. If the gain expansion in the transfer curve 15 would be matched perfectly, then the input-output characteristic 35 of the predistorter and amplifier chain would be linear.
The predistorter 20 can be programable. It can have, for example, a control input (not shown) to adjust its gain expansion parameter, or other parameters. In those cases the predistorter 20 will be used in conjunction with a control circuit (not shown) that will try to set the gain expansion parameter to a value that best cancels the nonlinearity of the main amplifier 10.
In direct digital synthesis, predistortion is done digitally and converted to an analog input with a digital-to-analog converter. In this case, the gain expansion parameter, or other parameters, can be varied only among a finite set of digital values.
A finite set of values of the adjustment parameters, however, limits the distortion correction. Distortion correction is strongly dependent on how close the gain expansion parameter matches an optimal value. At any given time, though, none of the discrete values of the set will exactly coincide with the optimal predistortion value needed. This optimal value most probably will fall in between a pair of values of the set. One value of the pair will under-correct the compression, while the other will over-correct the compression. Hence, either of these two values, may still result in significant uncorrected distortion. Thus, the distortion improvement is limited when there are only a limited number of predistortion values and the optimum value falls in between two values of the set. Although it is possible to increase the number of values, this approach increases the complexity and cost of the system. In some applications, the number of predistortion values should be as small as possible to minimize the size and power of the predistorter circuit while achieving high data and control bandwidth.
Conventional predistortion is an effective method for improving linearity, but requires accurate a priori prediction of the distortion occurring in the power amplifier. Typical adaptive predistortion requires detailed estimation and correction of the distorted transfer curve of the system and is limited by the correction bandwidth of the estimation loop. An example is shown in U.S. Pat. No. 6,587,514, by Wright et al., herein incorporated by reference in its entirety, which uses digital compensation signal processing in the estimation loop. This limits the correction bandwidth. In addition, it utilizes a large look-up table. This increases complexity, and can limit the speed of operation. What is needed is a circuit that does not require digital signal processing in the feedback loop. In addition, what is needed is a less complex circuit that does not require a large look-up table and excessive memory storage requirements for operation.